Tagged Questions

The special theory of relativity describes the motion and dynamics of objects moving at significant fractions of the speed of light.

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461 views

What happens if someone travelling just under the speed of light makes a telephone call?

Apart from the possibility that radio transmission will not work, what would happen if someone travelling fast enough relative to another individual to experience significant time dilation made a ...
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1answer
98 views

How can one always be standing still when compared to the speed of light?

I was thinking if I built a device with 7 clocks, synchronized to each other, one in the middle, one up, down, left, right, behind and in front of me, say 1 meter away, and I fired a laser from the ...
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2answers
566 views

Is special relativity a special case of general relativity, qualitatively?

Since Einstein name his theory Special Relativity and General Relativity, there should be some expected relationship between them, particularly "Special Relativity" being a special case of the more ...
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Lightspeed (invariance) measurement methods

I would like to know, how measurements of the speed of light are conducted these days, especially in the context of the invariance of $c$. Do all the methods involve mirrors to redirect the photons ...
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5answers
371 views

Special Relativity, 2nd Postulate — Why? [duplicate]

As a lowly physics undergrad who has been chewing on this 2nd postulate of special relativity for a year or more, I simply can't wrap my head around reasons why it is true or how Einstein might have ...
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2answers
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Special Relativity Question: Doppler shift

Imagine an observer watching a moving rocket carrying on it both, a light source and a clock. If on the rocket the clock is synchronized with the frequency of the light being emitted then will not a ...
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0answers
57 views

Spin in relativity

Mass and spin of the particle are used in classification of elementary particles. The mass is defined to be a Lorentz invariant quantity. On the other hand, the spin is a spacelike 4-vector and cannot ...
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1answer
75 views

Do photons change velocity instantaneously?

Any object traveling at c is observed as traveling at c in all reference frames. When a photon travels through a vacuum at c, all reference frames observe it traveling at c. When a photon passes ...
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2answers
162 views

A whole lot of doubts on Lorentz representation

Can someone tell me in layman's language how the $(1/2,1/2)$ represents a vector field and $(0,1/2)$ or $(1/2,0)$ represents spinors and $(0,0)$ represents scalar field. Please don't be pedantic on ...
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1answer
83 views

Thought experiment: Tethered galaxies - to the extreme

If two rocks were tied together with a tight, absurdly long, non-elastic rope, and placed on planets at either end of Earth's observable universe - or beyond - What would happen? Is the structural ...
2
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1answer
63 views

4-vector velocity to Newtonian?

The four-vector condition for a particle free of forces is: $\frac{du}{dτ} = 0$ and the equivalence of this to the statement of newton's first law follows from the expression for four-velocity: ...
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1answer
52 views

Relativistic Conservation laws [closed]

Conservation of Relativistic mass and thus energy is easily proven by considering an inelastic collision of two bodies while invoking the conservation of momentum. As such the momentum law appears ...
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2answers
82 views

Is negative mass for a bound system of two particles forbidden?

Is there any theorem that forbids the bound system of two massive particles to have negative mass?
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1answer
54 views

Doppler Shift when Light Travels Through Two Different Mediums

When considering the Doppler shift, the 'canonical equation' is $$f=\frac{c+vr}{c+vs}f_0$$ However, this equation seems to run into trouble in the following situation: A light source inside water is ...
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1answer
107 views

Minkowski spacetime vs Euclidian space

Two questions marked in bold: What is the magnitude of a Minkowski spacetime four velocity? I'm deducing that it is c for all observers, but I'd like it confirmed. In Euclidian space, a velocity ...
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2answers
110 views

Refractive index of dielectric in different frames of reference

The setup A transparent isotropic dielectric medium moving in the negative $x'$ direction at speed $v$ in frame $S'$ is stationary in frame $S$, where it has refractive index $n$. In other words, ...
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3answers
98 views

Coulomb's Law- Why the Coulomb's law is valid only for point and static charges?

Why the coulomb's law is valid only for point and static charges? Is there is any definite reason?
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1answer
498 views

Dark matter a medium for light propagation

Is dark matter a candidate to fill void left by luminiferous ether as a medium for light travel?
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4answers
3k views

Why is there a controversy on whether mass increases with speed?

Some people say that mass increases with speed, some people say that the mass of an object is independent of its speed. I understand how some (though not many) things in physics are a matter of ...
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5answers
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What problems with Electromagnetism led Einstein to the Special Theory of Relativity?

I have often heard it said that several problems in the theory of electromagnetism as described by Maxwell's equations led Einstein to his theory of Special Relativity. What exactly were these ...
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0answers
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Question about Origins in Galilean transformation

I'm just learning about relativity, and every equation I see for a galilean transformation of frame $S'$ (moving with uniform velocity in the $x$-direction with respect to frame $S$) is $x'=x-vt$, ...
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1answer
173 views

How do you go from quantum electrodynamics to Maxwell's equations?

I've read and heard that quantum electrodynamics is more fundamental than maxwells equations. How do you go from quantum electrodynamics to Maxwell's equations?
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2answers
295 views

Energy & Mass of a Photon [duplicate]

$$\text{Please read the whole question before answering}$$ Before I ask my question, I would like to say that "Yes, I do know a photon has no mass." I was helping someone here on P.SE with the ...
2
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1answer
92 views

Approaching speed of light: why do objects appear further away in front of me?

A Slower Speed of Light is a video game created by the MIT Game Lab which allows users to experience what it would be like if the speed of light was closer to normal walking/running speeds and thus ...
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1answer
181 views

How is the Photoelectric Effect affected by Blue-Shifting

I was thinking about the Photoelectric Effect and Blue-Shifting when I came up with a thought experiment that I couldn't think of an answer for. The thought experiment is as follows: A metal plate is ...
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2answers
94 views

Inertial frames

I'm just starting my study of relativity, and I have a rough understanding of the connection between inertial frames, newton's laws, and galilean transformations, but I'd probably benefit more if ...
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2answers
205 views

A curious case of Relativistic Velocity Addition [duplicate]

The relativistic velocity addition formula is $$u = \frac{v+u'}{1+ \frac{vu'}{c^2}}$$ Where $u$ = velocity of projectile seen by rest observer "A" $v$ = velocity of moving observer "B" as seen by ...
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4answers
422 views

Are Lorentz transformations linear transformations? [duplicate]

My textbook says that Lorentz transformations are linear transformations and present them as matrices. Lorentz transformations relate different coordinate systems with each other. It seems that ...
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1answer
78 views

Prove that a derivative with respect to a covariant 4-vector is a contravariant vector operator

In special relativity, I know you can prove that the derivative with respect to a contravariant 4-vector component transforms like a covariant vector operator by using the chain rule, but I can't work ...
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1answer
107 views

Is light so important in special relativity?

I'm an amateur physics enthusiast with no formal university education in Physics. So my question might sound very naive, so forgive me. I had this question in the back of my mind since the wrong ...
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3answers
102 views

Speed greater than light in circular motion [duplicate]

We know that linear speed of object going around a circle is $\omega * r $ Now let us take an elastic string and rotate a body of negligible mass with $\omega = 500 rad/s$ It is possible to further ...
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2answers
64 views

Would an object lose physical mass if it accelerated to a relativistic speed (would an object burn it's own mass)?

First of all, I have read other questions, and seen how an object gains mass just by gaining a lot of speed. But here, I am talking about losing some of that mass to convert the value into kinetic ...
2
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3answers
65 views

Is sound relative when travelling fast?

If I was listening to music through my headphones and travelling close to the speed of light and time is slowing down around me , would the music slow down as well?
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1answer
84 views

Significance of angle in relativistic mass equation

I was playing with $$M=\frac{m_0}{\sqrt{1-v^2/c^2}}$$ and when I put $v=c\sin\theta$, I got $$M=m_0\sec\theta.$$ And this new equation is appearing so beautiful and attracting to me that I was ...
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2answers
180 views

Is time nothing but the speed of light (or the light itself)?

With regard to relativistic effects on time, all the examples and explanations revolve around light and its speed. Especially in explanatory situations that explain this using photon clock, it seems ...
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3answers
164 views

Does this count as moving faster than light?

I'm not familiar with any complicated physics equation, however I do understand some basics. Suppose there is two objects, both of them are moving away from each other in a 3-dimensional space, which ...
2
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3answers
130 views

Can lightly-ionized atoms be accelerated to relativistic speeds with current technology?

I know there're lots of various particle accelerators, which can accelerate particles to TeV's of energy, but it seems they all work on electrons or nuclei or other elementary of tightly-bound ...
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1answer
51 views

Universal speed limit [duplicate]

Is there any reason the universe has matter not being able to exceed the speed of light, or why there is a speed limit in the first place? I know why it can't, meaning the basic physics of it. I ...
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2answers
88 views

How do we know that clocks slow down relative to each other? [duplicate]

For example if a body in motion experiences time dilation, why does it see a body at rest slow down relative to it? wouldn't it make more sense if the body at rest has a faster clock relative to it?
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2answers
51 views

Magnetic force and relative frame

The magnetic field for to a moving charge depend on its velocity (Biot Savart's law). My question is that is it then not frame dependent? If it is, it means if a man is walking and other is standing ...
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1answer
70 views

If the mass of an object or system is a measure of its energy content, multiplied by speed of light squared…?

If the mass of an object or system is a measure of its energy content, multiplied by speed of light squared, does it mean that the total potential energy of the rest mass of object or entire system ...
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4answers
170 views

What is the intuition behind the Lorentz factor from Special Relativity

Time Dilation = $1/\sqrt{ 1-v^2/c^2}$ but why? How do you get to that conclusion? I know you use Pythagoras'theorem and my current understand goes like this: $$vt^2+ct^2=cT^2$$ then you take the ...
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0answers
50 views

Deformation of light-cone

In the paper The geometry of free fall and light propagation by Ehlers and his colleagues (Gen. Relativ. Gravit. 44 no. 6, pp. 1587–1609 (2012)), when the authors introduce the differentiable ...
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2answers
85 views

Consider relativity. If a stick accelerates from a stationary state, how will it look like during the acceleration?

I mean how it will be shortened in the sight of an observer in a stationary coordinate system. It seems I was not clear enough. If the stick becomes shorter, then while this is happening, the ...
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3answers
524 views

Physical reason for Lorentz Transformation

Seeing the mathematical derivation of the Lorentz Transformation for time coordinates of an event for two observers we get the term. Now how to make sense physically of the $t-\frac{vx}{c^2}$ ...
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0answers
61 views

What is the derivation of the speed of light $c$ that is not based on electromagnetism? [duplicate]

The "speed of light" is not just the speed of electromagnetic radiation, but of any massless particle. Therefore must not there be an expression for $c$ that is not in terms of $\mu_0$ and ...
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6answers
6k views

Can you run away from your shadow?

Now this might be a silly question but it's actually bugging me, this one might be easier to understand if you have kids that watch (or used to watch) Peppa Pig. In one of the episodes, about shadows, ...
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0answers
39 views

Conserved charge from conserved current associated with translational invariance

(c.f Di Francesco, 'Conformal Field Theory' P.45) Di Francesco calls the conserved charge arising from the conserved current associated with a translation invariant theory the 'four momentum'. While ...
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1answer
52 views

If we could reach very high speeds, could we measure the velocity of the Earth this way?

If we could reach (nearly) the speed of light with spaceships, could we measure the velocity of the Earth by launching three perpendicular rockets, accelerating them, and measuring how much fuel ...
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1answer
42 views

What is the status of massless photons traveling through a medium?

Photons in vacuum have no proper time, and they are not considered as observers and not as reference frame. But what about photons travelling through matter? Their velocity is lower than light speed, ...